Re: [PATCH v3 04/11] mm: vmalloc: Remove global vmap_area_root rb-tree

[Uladzislau Rezki]

Hello, Wen Gu.

> 
> Hi Uladzislau Rezki,
> 
> I really like your work, it is great and helpful!
> 
> Currently, I am working on using shared memory communication (SMC [1])
> to transparently accelerate TCP communication between two peers within
> the same OS instance[2].
> 
> In this scenario, a vzalloced kernel buffer acts as a shared memory and
> will be simultaneous read or written by two SMC sockets, thus forming an
> SMC connection.
> 
> 
>           socket1                    socket2
>              |                          ^
>              |                          |    userspace
>       ---- write -------------------- read ------
>              |    +-----------------+   |    kernel
>              +--->|  shared memory  |---+
>                   | (vzalloced now) |
>                   +-----------------+
> 
> Then I encountered the performance regression caused by lock contention
> in find_vmap_area() when multiple threads transfer data through multiple
> SMC connections on machines with many CPUs[3].
> 
> According to perf, the performance bottleneck is caused by the global
> vmap_area_lock contention[4]:
> 
> - writer:
> 
>   smc_tx_sendmsg
>   -> memcpy_from_msg
>      -> copy_from_iter
>         -> check_copy_size
>            -> check_object_size
>               -> if (CONFIG_HARDENED_USERCOPY is set) check_heap_object
>                  -> if(vm) find_vmap_area
>                     -> try to hold vmap_area_lock lock
> - reader:
> 
>   smc_rx_recvmsg
>    -> memcpy_to_msg
>       -> copy_to_iter
>          -> check_copy_size
>             -> check_object_size
>                -> if (CONFIG_HARDENED_USERCOPY is set) check_heap_object
>                   -> if(vm) find_vmap_area
>                      -> try to hold vmap_area_lock lock
> 
> Fortunately, thank you for this patch set, the global vmap_area_lock was
> removed and per node lock vn->busy.lock is introduced. it is really helpful:
> 
> In 48 CPUs qemu environment, the Requests/s increased by 5 times:
> - nginx
> - wrk -c 1000 -t 96 -d 30 http://127.0.0.1:80
> 
>                 vzalloced shmem      vzalloced shmem(with this patch set)
> Requests/sec          113536.56            583729.93
> 
> 
Thank you for the confirmation that your workload is improved. The "nginx"
is 5 times better!

> But it also has some overhead, compared to using kzalloced shared memory
> or unsetting CONFIG_HARDENED_USERCOPY, which won't involve finding vmap area:
> 
>                 kzalloced shmem      vzalloced shmem(unset CONFIG_HARDENED_USERCOPY)
> Requests/sec          831950.39            805164.78
> 
> 
The CONFIG_HARDENED_USERCOPY prevents coping "wrong" memory regions. That is 
why if it is a vmalloced memory it wants to make sure it is really true,
if not user-copy is aborted.

So there is an extra work that involves finding a VA associated with an address.

> So, as a newbie in Linux-mm, I would like to ask for some suggestions:
> 
> Is it possible to further eliminate the overhead caused by lock contention
> in find_vmap_area() in this scenario (maybe this is asking too much), or the
> only way out is not setting CONFIG_HARDENED_USERCOPY or not using vzalloced
> buffer in the situation where cocurrent kernel-userspace-copy happens?
> 
Could you please try below patch, if it improves this series further?
Just in case:

<snip>
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index e30dabf68263..40acf53cadfb 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -772,7 +772,7 @@ static DEFINE_PER_CPU(struct vmap_area *, ne_fit_preload_node);
 struct rb_list {
 	struct rb_root root;
 	struct list_head head;
-	spinlock_t lock;
+	rwlock_t lock;
 };
 
 struct vmap_pool {
@@ -947,19 +947,19 @@ find_vmap_area_exceed_addr_lock(unsigned long addr, struct vmap_area **va)
 	for (i = 0; i < nr_vmap_nodes; i++) {
 		vn = &vmap_nodes[i];
 
-		spin_lock(&vn->busy.lock);
+		read_lock(&vn->busy.lock);
 		va_lowest = __find_vmap_area_exceed_addr(addr, &vn->busy.root);
 		if (va_lowest) {
 			if (!va_node || va_lowest->va_start < (*va)->va_start) {
 				if (va_node)
-					spin_unlock(&va_node->busy.lock);
+					read_unlock(&va_node->busy.lock);
 
 				*va = va_lowest;
 				va_node = vn;
 				continue;
 			}
 		}
-		spin_unlock(&vn->busy.lock);
+		read_unlock(&vn->busy.lock);
 	}
 
 	return va_node;
@@ -1695,9 +1695,9 @@ static void free_vmap_area(struct vmap_area *va)
 	/*
 	 * Remove from the busy tree/list.
 	 */
-	spin_lock(&vn->busy.lock);
+	write_lock(&vn->busy.lock);
 	unlink_va(va, &vn->busy.root);
-	spin_unlock(&vn->busy.lock);
+	write_unlock(&vn->busy.lock);
 
 	/*
 	 * Insert/Merge it back to the free tree/list.
@@ -1901,9 +1901,9 @@ static struct vmap_area *alloc_vmap_area(unsigned long size,
 
 	vn = addr_to_node(va->va_start);
 
-	spin_lock(&vn->busy.lock);
+	write_lock(&vn->busy.lock);
 	insert_vmap_area(va, &vn->busy.root, &vn->busy.head);
-	spin_unlock(&vn->busy.lock);
+	write_unlock(&vn->busy.lock);
 
 	BUG_ON(!IS_ALIGNED(va->va_start, align));
 	BUG_ON(va->va_start < vstart);
@@ -2123,10 +2123,10 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end,
 		if (RB_EMPTY_ROOT(&vn->lazy.root))
 			continue;
 
-		spin_lock(&vn->lazy.lock);
+		write_lock(&vn->lazy.lock);
 		WRITE_ONCE(vn->lazy.root.rb_node, NULL);
 		list_replace_init(&vn->lazy.head, &vn->purge_list);
-		spin_unlock(&vn->lazy.lock);
+		write_unlock(&vn->lazy.lock);
 
 		start = min(start, list_first_entry(&vn->purge_list,
 			struct vmap_area, list)->va_start);
@@ -2223,9 +2223,9 @@ static void free_vmap_area_noflush(struct vmap_area *va)
 	vn = is_vn_id_valid(vn_id) ?
 		id_to_node(vn_id):addr_to_node(va->va_start);
 
-	spin_lock(&vn->lazy.lock);
+	write_lock(&vn->lazy.lock);
 	insert_vmap_area(va, &vn->lazy.root, &vn->lazy.head);
-	spin_unlock(&vn->lazy.lock);
+	write_unlock(&vn->lazy.lock);
 
 	trace_free_vmap_area_noflush(va_start, nr_lazy, nr_lazy_max);
 
@@ -2272,9 +2272,9 @@ struct vmap_area *find_vmap_area(unsigned long addr)
 	do {
 		vn = &vmap_nodes[i];
 
-		spin_lock(&vn->busy.lock);
+		read_lock(&vn->busy.lock);
 		va = __find_vmap_area(addr, &vn->busy.root);
-		spin_unlock(&vn->busy.lock);
+		read_unlock(&vn->busy.lock);
 
 		if (va)
 			return va;
@@ -2293,11 +2293,11 @@ static struct vmap_area *find_unlink_vmap_area(unsigned long addr)
 	do {
 		vn = &vmap_nodes[i];
 
-		spin_lock(&vn->busy.lock);
+		write_lock(&vn->busy.lock);
 		va = __find_vmap_area(addr, &vn->busy.root);
 		if (va)
 			unlink_va(va, &vn->busy.root);
-		spin_unlock(&vn->busy.lock);
+		write_unlock(&vn->busy.lock);
 
 		if (va)
 			return va;
@@ -2514,9 +2514,9 @@ static void free_vmap_block(struct vmap_block *vb)
 	BUG_ON(tmp != vb);
 
 	vn = addr_to_node(vb->va->va_start);
-	spin_lock(&vn->busy.lock);
+	write_lock(&vn->busy.lock);
 	unlink_va(vb->va, &vn->busy.root);
-	spin_unlock(&vn->busy.lock);
+	write_unlock(&vn->busy.lock);
 
 	free_vmap_area_noflush(vb->va);
 	kfree_rcu(vb, rcu_head);
@@ -2942,9 +2942,9 @@ static void setup_vmalloc_vm(struct vm_struct *vm, struct vmap_area *va,
 {
 	struct vmap_node *vn = addr_to_node(va->va_start);
 
-	spin_lock(&vn->busy.lock);
+	read_lock(&vn->busy.lock);
 	setup_vmalloc_vm_locked(vm, va, flags, caller);
-	spin_unlock(&vn->busy.lock);
+	read_unlock(&vn->busy.lock);
 }
 
 static void clear_vm_uninitialized_flag(struct vm_struct *vm)
@@ -4214,19 +4214,19 @@ long vread_iter(struct iov_iter *iter, const char *addr, size_t count)
 
 	next_va:
 		next = va->va_end;
-		spin_unlock(&vn->busy.lock);
+		read_unlock(&vn->busy.lock);
 	} while ((vn = find_vmap_area_exceed_addr_lock(next, &va)));
 
 finished_zero:
 	if (vn)
-		spin_unlock(&vn->busy.lock);
+		read_unlock(&vn->busy.lock);
 
 	/* zero-fill memory holes */
 	return count - remains + zero_iter(iter, remains);
 finished:
 	/* Nothing remains, or We couldn't copy/zero everything. */
 	if (vn)
-		spin_unlock(&vn->busy.lock);
+		read_unlock(&vn->busy.lock);
 
 	return count - remains;
 }
@@ -4563,11 +4563,11 @@ struct vm_struct **pcpu_get_vm_areas(const unsigned long *offsets,
 	for (area = 0; area < nr_vms; area++) {
 		struct vmap_node *vn = addr_to_node(vas[area]->va_start);
 
-		spin_lock(&vn->busy.lock);
+		write_lock(&vn->busy.lock);
 		insert_vmap_area(vas[area], &vn->busy.root, &vn->busy.head);
 		setup_vmalloc_vm_locked(vms[area], vas[area], VM_ALLOC,
 				 pcpu_get_vm_areas);
-		spin_unlock(&vn->busy.lock);
+		write_unlock(&vn->busy.lock);
 	}
 
 	/*
@@ -4687,7 +4687,7 @@ bool vmalloc_dump_obj(void *object)
 
 	vn = addr_to_node((unsigned long)objp);
 
-	if (spin_trylock(&vn->busy.lock)) {
+	if (read_trylock(&vn->busy.lock)) {
 		va = __find_vmap_area(addr, &vn->busy.root);
 
 		if (va && va->vm) {
@@ -4697,7 +4697,7 @@ bool vmalloc_dump_obj(void *object)
 			success = true;
 		}
 
-		spin_unlock(&vn->busy.lock);
+		read_unlock(&vn->busy.lock);
 	}
 
 	if (success)
@@ -4742,13 +4742,13 @@ static void show_purge_info(struct seq_file *m)
 	for (i = 0; i < nr_vmap_nodes; i++) {
 		vn = &vmap_nodes[i];
 
-		spin_lock(&vn->lazy.lock);
+		read_lock(&vn->lazy.lock);
 		list_for_each_entry(va, &vn->lazy.head, list) {
 			seq_printf(m, "0x%pK-0x%pK %7ld unpurged vm_area\n",
 				(void *)va->va_start, (void *)va->va_end,
 				va->va_end - va->va_start);
 		}
-		spin_unlock(&vn->lazy.lock);
+		read_unlock(&vn->lazy.lock);
 	}
 }
 
@@ -4762,7 +4762,7 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
 	for (i = 0; i < nr_vmap_nodes; i++) {
 		vn = &vmap_nodes[i];
 
-		spin_lock(&vn->busy.lock);
+		read_lock(&vn->busy.lock);
 		list_for_each_entry(va, &vn->busy.head, list) {
 			if (!va->vm) {
 				if (va->flags & VMAP_RAM)
@@ -4808,7 +4808,7 @@ static int vmalloc_info_show(struct seq_file *m, void *p)
 			show_numa_info(m, v);
 			seq_putc(m, '\n');
 		}
-		spin_unlock(&vn->busy.lock);
+		read_unlock(&vn->busy.lock);
 	}
 
 	/*
@@ -4902,11 +4902,11 @@ static void vmap_init_nodes(void)
 		vn = &vmap_nodes[n];
 		vn->busy.root = RB_ROOT;
 		INIT_LIST_HEAD(&vn->busy.head);
-		spin_lock_init(&vn->busy.lock);
+		rwlock_init(&vn->busy.lock);
 
 		vn->lazy.root = RB_ROOT;
 		INIT_LIST_HEAD(&vn->lazy.head);
-		spin_lock_init(&vn->lazy.lock);
+		rwlock_init(&vn->lazy.lock);
 
 		for (i = 0; i < MAX_VA_SIZE_PAGES; i++) {
 			INIT_LIST_HEAD(&vn->pool[i].head);
<snip>

Thank you!

--
Uladzislau Rezki